Field of the Invention
This invention relates to time-to-digital converters and more particularly to addressing offset issues in time-to-digital converters.
Description of the Related Art
Time-to-digital converters (TDCs) can be found in such circuits as phase-locked loops (PLLs) and frequency-locked loops (FLLs). FIG. 1A illustrates a high level block diagram of a conventional fractional-N PLL 100 that utilizes a time-to-digital converter 101. The time-to-digital converter 101 generates a digital value corresponding to a difference between a reference clock signal 103 and a feedback clock signal 105 and supplies the digital value to digital loop filter 107. The digital loop filter controls the digitally controlled oscillator (DCO) 109 to generate an output clock signal 111.
FIG. 1B illustrates a high level block diagram of a frequency-locked loop (FLL) 150 that utilizes a frequency-to-digital converter 121, which is another type of time-to-digital converter where the time represents a frequency difference. As used herein, TDC refers to a circuit that quantizes time related information, e.g., frequency, phase, or absolute time. The frequency-to-digital converter 121 generates a digital value corresponding to a frequency difference between a reference clock signal 123 and an output clock signal 125 and supplies the digital value to digital loop filter 127. The digital loop filter controls the digitally controlled oscillator (DCO) 129 to generate the output clock signal 125 with the desired frequency.
There are generally two types of circuits useful for quantizing time that can be used in a time-to-digital converter. The first type is “counter-based” and counts the number of reference clock edges that occur during a given time. The counter-based TDC can measure arbitrarily long intervals limited only by the size of the counter, but the resolution of the counter-based TDC is limited by the clock frequency doing the counting. The second type of TDC is “pulse-based” and measures the time of a pulse by converting the pulse duration to another analog quantity (such as voltage). For example, the pulse may be used to charge a capacitor. The voltage across the capacitor may then be converted to a digital value in an analog to digital converter. The resolution of the pulse-based TDC can be reduced below a clock period, but the measurable interval is limited and usually small.
To get the advantages of both types of TDCs, counter-based time-to-digital converter and the pulse-based time-to-digital converter can be combined into a coarse-fine quantizer. The counter-based type forms the coarse quantizer and pulse-based type forms the fine quantizer. If it is functioning properly, the resolution will be the same as the fine quantizer (very good), but the range will be that of the coarse quantizer (arbitrarily large). The problem is that any mismatch in the instant at which the two quantizers take their time sample can reduce the resolution to that of the coarse quantizer, which makes implementation of such circuits problematic.